The present invention relates to a liquid level detector, and more particularly, to a liquid level detector which automatically detects a liquid level as a remaining amount of liquid stored in a transportation fuel tank of a car, an airplane or the like.
Conventionally, as an apparatus for detecting a liquid level in a fuel tank of a car, there has been known a liquid level detector in which a float arm is slid on a resistance plate by a float vertically moving according to a liquid level, and the liquid level is converted into a potential difference to detect the height of liquid.
An example of the liquid level detector will be described. FIG. 1 is an electric block diagram for explaining a structural example of a sensor used in a liquid level detector according to the related art and the present invention. FIG. 2 is an explanatory diagram for explaining a structural example of the liquid level detector according to the related art and the present invention. FIG. 3 is an explanatory diagram for explaining a structural example of a variable resistor in the sensor according to the related art and the present invention.
A sensor 2 of a liquid level detector 1 includes a variable resistor 3 which varies its resistance as contact points 19 and 20 move according to the change in height of liquid in a tank T of an airtight container. The variable resistor 3 is connected in series to a fixed resistor 7. The variable resistor 3 and the fixed resistor 7 are connected to a power circuit 4 which applies a specific voltage thereto.
The sensor 2 includes, as shown in FIGS. 2 and 3, a resistance plate 13 attached to a main body frame 12, and a sliding contact 14 connected to a float arm 11. In this case, a float 10 is floating on a liquid surface by the buoyancy of the liquid and is attached to a front end of the float arm 11, and the sliding contact 14 is connected to the other end of the float arm 11. The resistance plate 13 of the sensor 2 is provided with a first conductive pattern 15 and a second conductive pattern 16. The two conductive patterns 15 and 16 are arranged in parallel to each other in a circular arc shape centering on a rotational shaft 21 of the float arm 11. An input/output conductive portion 17 is connected to one end of the first conductive pattern 15, and an input/output conductive portion 18 is connected to one end of the second conductive pattern 16.
The first conductive pattern 15 includes a plurality of conductive segments 15a arranged at predetermined intervals in a circumferential direction of the circular arc-shaped pattern, and a resistor 15b electrically connecting the conductive segments 15a to each other. Further, the second conductive pattern 16 includes a plurality of conductive segments 16a arranged at predetermined intervals in a circumferential direction of the circular arc-shaped pattern, and a connector 16b electrically connecting the conductive segments 16a to each other.
The sliding contact 14 has two contact points 19 and 20 electrically connected to each other. Further, the sliding contact 14 is connected to the rotational shaft 21 provided at the other end of the float arm 11. The float arm 11 moves downward according to the amount consumed from a liquid level of the float 10 floating on the liquid surface in a full state, so that the float arm 11 is rotated in a direction of arrow Y of FIG. 3 with respect to the rotational shaft 21. Accordingly, the sliding contact 14 is also rotated in the direction of arrow Y of FIG. 3 with the rotation of the float arm 11. By the rotation of the sliding contact 14, the contact points 19 and 20 slide on and come into electrical contact with the conductive segments 15a and 16a arranged on the first conductive pattern 15 and the second conductive pattern 16 respectively. Accordingly, the length of the resistor 15b interposed between the input/output conductive portion 17 connected to the first conductive pattern 15 and the input/output conductive portion 18 connected to the second conductive pattern 16 is changed, thereby changing a resistance of a circuit interposed between the input/output conductive portions 17 and 18 (i.e., a resistance of the variable resistor 3 of FIG. 1). The first conductive pattern 15, the second conductive pattern 16 and the sliding contact 14 constitute the variable resistor 3.
When a voltage is applied to the variable resistor 3, the sensor 2 detects a potential difference between the input/output conductive portions 17 and 18, and outputs an output signal to a processing circuit 5. The processing circuit 5 calculates a remaining amount of liquid on the basis of the output signal of the sensor 2 and the result thereof is displayed in a bar graph or analog form on a display such as a meter 6. Further, the meter 6 may include a fixed resistor provided in a line connected to the processing circuit 5.
In the aforementioned liquid level detector, the contact points are generally formed of an alloy of silver (Ag) and palladium (Pd), an alloy of silver (Ag) and copper (Cu), an alloy of silver (Ag) and nickel (Ni) or the like. Further, the conductive segments are formed of, e.g., a mixture of Ag—Pd powder and glass, which is obtained by mixing silver power, palladium power and glass powder into paste, printing the paste on the resistance plate, and then drying and sintering the paste.
However, the liquid level detector may be used in a fuel tank of a car using, as a fuel, an electrolytic solution (alcohol) such as ethanol and methanol, or gasoline including the electrolytic solution. Since silver (Ag) has a low electric resistance, silver exhibits excellent conductivity. However, the contact points or the conductive segments formed of silver may be degraded or corroded due to sulfur, water, alcohol and the like included in the fuel. As a result, conduction failure may occur to thereby make measurement impossible or cause measurement errors. Accordingly, in order to prevent the conductive segments or the contact points from being degraded or corroded, there has been proposed a technology for mixing gold (Au) with a material of the conductive segments or the contact points, thereby improving degradation resistance and corrosion resistance (see, e.g., Patent Documents 1 and 2).                Patent Document 1: JP-A-2002-202179        Patent Document 2: JP-A-2003-287457        
However, in case of using a gold alloy to ensure the degradation resistance and corrosion resistance, the gold content is required to be equal to or greater than about 40% by mass in the conductive segments and to be equal to or greater than about 98% by mass in the contact points in order to obtain a sufficient effect. In addition, the use of expensive gold leads to an increase in cost.